The ilium is a significant bone in the human body, forming a substantial part of the pelvic structure. It is one of the three bones that fuse to create the hip bone (also known as the coxal bone or os coxa). This bone plays an important role in supporting the upper body and facilitating movement.
Understanding the Ilium
The ilium is the largest and uppermost component of the hip bone, contributing to the pelvic girdle. Its shape is often described as fan-like or blade-like, expanding superiorly to form a broad surface. In adults, it fuses with the ischium and pubis to form a single, robust hip bone.
It connects posteriorly with the sacrum, a triangular bone at the base of the spine. This connection forms the largely immobile sacroiliac joint, providing stability to the pelvis. The ilium also contributes to the acetabulum, a cup-shaped socket that articulates with the head of the femur to form the hip joint.
Key Anatomical Features
The ilium features several landmarks important for anatomical reference. The most prominent is the iliac crest, the thickened, curved superior border of the bone, extending from front to back.
At the anterior end of the iliac crest is the anterior superior iliac spine (ASIS), a palpable bony projection. Below the ASIS is the anterior inferior iliac spine (AIIS). Posteriorly, the iliac crest terminates at the posterior superior iliac spine (PSIS). Inferior to the PSIS is the posterior inferior iliac spine (PIIS).
On its internal surface is the iliac fossa, a large, smooth, concave area. This depression is part of the expanded upper portion, or wing. These points and surfaces provide shape to the pelvic region.
Roles and Importance
The ilium serves multiple roles. A primary function is supporting the upper body and transmitting forces from the spine to the lower limbs, allowing for upright posture and locomotion.
Its expansive surfaces provide attachment sites for numerous muscles. These muscles, including those of the abdominal wall, gluteal region, and thigh, maintain posture, facilitate movement, and ensure body stability. The ilium also forms part of the pelvic cavity, protecting internal organs like parts of the digestive and reproductive systems.
Beyond its structural and muscular functions, the iliac crest is a site of biological activity. It contains a rich supply of bone marrow, making it a common source for transplants. This highlights its broader contribution to the body’s systems, extending beyond its skeletal role.